In a conventional digital radio communication system, the TDMA method is used for the both communications of down and up lines between a base station and a mobile station selected from a plurality of mobile stations. The base station comprises a TDMA transmitter, a TDMA receiver, and a timing generator, and the mobile station comprises a TDMA receiver and a TDMA transmitter.
In operation, the timing generator of the base station generates a timing signal defining frames, and the TDMA transmitter of the base station transmits a high frequency transmitting signal having, for instance, six slots (bursts) to provide six channels via the down line to the mobile station, so that the TDMA receiver of the mobile station receives the burst signal corresponding to, for instance, the channel 2 in the transmitted high frequency signal, and the TDMA transmitter of the mobile station transmits a high frequency transmitting signal corresponding to the channel 2 at a specified time via the up line to the base station, after the TDMA receiver of the mobile station receives the transmitted signal. Then, the TDMA receiver of the base station receives the signal of the channel 2 transmitted from the base station at this specified time. In this operation, the synchronism is set up in accordance with the relation between the timing signal and the specified time.
In the conventional digital radio communication system, the synchronism in the communication between the base and mobile stations may be set up by using a method described in the Japanese Patent Kokai Nos. 64-84927 and 1-181336. In such a method, a transmitting time is controlled by calculating a communication distance between base and mobile stations, or in accordance with a time, at which a unique key word included in a received burst signal is detected.
Recently, the CDMA method utilizing spread spectrum technology is practically used in place of the TDMA method in digital mobile radio communication system. In this CDMA method, channels are defined by plural spread spectrum codes having no correlation with each other.
In case where this CDMA method is used for a down line, in which radio communication is carried out with complete synchronism, there are expected advantages set out below:
(a) it is possible to demodulate a received signal in a mobile station without interference among channels, and
(b) the influence of multi-path fading is neglected substantially.
In case where the CDMA method is used for a up line, however, there are expected disadvantages set out below:
(a) the difference occurs in electric field strengths of received signals from mobile stations, because the distance between the base station and each selected mobile station among a plurality of mobile stations, so that communication is deteriorated between the base station and a mobile station having a long distance therefrom, although this is avoided by precisely controlling a transmitting power in the mobile station, and
(b) there is difference in times, at which transmitting signals from mobile stations are received in the base station, so that it is difficult to set up the synchronism of spread spectrum code signals, thereby necessitating a complicated demodulation method.
In accordance with this study, it is considered that a mixed type of a digital mobile communication system in which the CDMA method is used for a down line, and the TDMA method is used for a up line is adopted to overcome the above disadvantages.
In the mixed type of a digital mobile radio communication system, however, there is a disadvantage in that a question as to how a timing for transmission of a transmitting burst signal of the TDMA method in a mobile station is determined is raised, because it is difficult for the mobile station to determine the timing by receiving a transmitted signal which is continuous in regard to time in the CDMA method.